Prior art magnetic coating compositions have been fabricated using both thermoplastic and thermosetting binder systems. Although many of the prior art binder systems exhibit those desirable characteristics for their particular intended uses, most prior art systems exhibit poor durability as measured by either head crash resistance or their ability to withstand physical abrasion. Further, most prior art binder systems exhibit cratering, pinholes, discontinuities in the coating, low signal amplitude, bit-shift, poor overwrite, low resolution, poor signal to noise ratio, poor surface finish, high switching field distribution, low orientation ratios, as well as low magnetic squareness and poor dispersion quality.
Generally, in order to improve one or more of the physical, electrical or magnetic characteristics of the coating, it has been necessary to compromise certain characteristics, the choice being dependent upon the final end use of the coating.
There are many reasons for the above mentioned deficiencies in prior binder compositions. One of the most important is poor dispersion of the magnetic oxide in the coating. For example, the phosphate ester described in this invention produces improved dispersions when compared to other prior art surfactants. Thus, improved magnetic oxide dispersion results in enhanced coating continuity and surface smoothness, which in turn results in an increase in signal amplitude, improved signal to noise ratio and superior magnetic parameters including orientation ratio, switching field distribution and squareness. It was also learned that excessive bit-shift is due to poor dispersion. In addition, most prior art binder systems incorporate a catalyst system based on an amine which cures through the epoxy groups. Solid epoxy resins claimed in prior art have a high epoxy equivalent which limits the amount of cross linking that can occur. If, however, a Lewis-Acid type catalyst such as an acidic phosphate ester is used, cross linking occurs between the epoxy and phenolic resins which leads to a three dimensional network with improved durability.
It is thus an object of the present invention to provide a magnetic coating composition without the above-recited deficiencies of prior art compositions.
It is yet another object of the present invention to provide a magnetic coating composition which exhibits enhanced dispersion quality characteristics over those compositions of the prior art.
It is yet another object of the present invention to provide a magnetic coating composition exhibiting greater durability, fewer discontinuities, greater signal amplitude as well as improved bit-shift, overwrite, resolution, signal to noise, switching field distribution and magnetic squareness characteristics.
It is still another object of the present invention to provide a magnetic coating composition with an improved dispersant to perform those objects as recited above.